Sinful Nature
by Blue Fire Yukio
Summary: When Rin figures out the secret behind Yukio's tail, he falls into the sinful nature of lust and allows satan to be able to take form to them.


Sinful Nature

A Blue Exorcist AU by PupsIsMyDog

(Sinful Nature is when a person comits a sin and lets a demon related to that sin into the persons life. The person who lets it in is the only person who can rid of them)

* * *

Rin and his dear younger brother Yukio were winding down on a fine

Sunday evening. Today was the only day of the week where they had to

do absolutly nothing. No homework. No exorcisms unless they were

direly needed. No appointments or classes. Just one entire 24 period

when they could stop to enjoy life and talk like the brothers they

were. Technical civilized chats werent used at these times they had

together. But this time was a bit different than other Sundays.

Rin had been thinking about something all week to ask Yukio, but only

now did it seem right to ask. "Yuki" he began casually, "Why do you

always hide your tail?" Yukio shrugged at the question and put his

manga aside. "A demons weakness is his tail, so I try to keep it

concealed, as uncomfortable as it is" answered the younger sibling.

Rin frowned a bit a flicked his own tail. It was free. So free he

oftentimes forgot he had a tail. Yukio must be so uncomfortable; no

wonder he's so distressed all the time. Not only did he have heavy

responsibilities on his shoulders, but he carried out his duties in an uncomfortable state.

Rin got up and walked over to his brother, reaching in the back

of his pants. "Its Sunday. The one day we're free. So we should be

able to feel free too" He pulled out the tail, but let go instantly

when Yukio reacted. His face was flustered and he held a hand over his

mouth. "R-Rin, be more careful with it!" he whined. Rin pulled it the

rest of the way out gingerly, being sure to touch it as carefully as he

could. Once out, Yukios tail was nicer than expected. Probably because

it didnt hang out all the time like Rin's to give it a chance to get dirty or matted. The older brother smiled and petted the tail as if it were Kuro. Yukio reacted fast with a

sharp moan as he pulled his tail away. "Leave it alone Rin! Its

sensitive!" He barked. Rin backed away. He didnt bother his brothers

tail any more that day. But curiosity about it brewed in his mind all

week. What did Yukio mean by sensitive? Was he hurt? As the older

brother, it was Rin's job to find out.

All week Rin concentrated on how he would do this. He couldnt be too

passive, or Yukio wouldnt allow him to inspect. However, he didnt want

to seem too demanding either. When Sunday came around again and church

was over, he bagan his loose plan.

They sat across from eachother, reading manga or playing on their

phones as usual. About an hour into it, Rin put his phone aside and

looked at Yukio sadly. Once again his tail was trapped on a free day.

Just like last week, Rin walked over and started pulling it out. "What

did I tell you!? Its sensitive!" He shouted crossly. But Rin didnt

care. He inspected the tail, looking for ways that it might be hurt.

"It looks fine, how is it sensitive?" He alast asked. He rubbed the

tail softly, and only then did he get an answer. "It makes me all weak

and tingly inside, dang it!" Yukio half-whined. This somehow turned

something on inside Rin. Was Yukio's tail really an e-zone? He could

do something with this. He could tie Yukio down and force him to what

he wanted. He might not ever need to do homework again! And Yukio would either have to comply or else-Wait! Where were these thoughts coming from? They

were brothers! Yukio wasnt subject to Rin's every whim-but he could

be. Very easily.

Rin fought back and forth to the two voices. One told him this was

wrong; his sinful nature told him it was right. And, sadly, it was his

sinful nature on his demon side that won the battle. He jerked Yukio

out of his seat, red shine glowing from the depths of his eyes.

"Please Rin! Let me go! Let me go!" Yukio begged fearfully, but he was

no match for the stronger sibling. Yukio was very scared, and he had every reason

to be. Rin had never done anything close to this to anyone before. Rin began to take on his demon form as he pinned his brother. There were no flames, but his ears and fangs

had grown longer. His elongated tongue licked hungrily on his own

lips. Drool dripped from his mouth as he stroked Yukio's face with a single

sharp claw.

"R-Rin! Please! Please dont hurt me!" Yukio said at the

brink of tears. Rin only loosed a low growl and dug his nails into Yukio's arms. Blood dripped down his white shirt. "Now give me that pretty little tail of yours!" Rin

growled. Yukio shook his head and wrapped his tail tightly around his

leg. Rin hissed crossly, but smiled. He ran a claw across the surface

of the black tail, making Yukio cry out. "S-Stop! Im asking

you Rin! I know ive been hard on you, but I promise to better if you

let me go! Dont fall into your sinful nature! It will let other demons in, and God knows which ones!" Rin bursted into laughter at this. "Dear brother, demons cant always be on their best behaviour! And about being hard on me? Well youre doing that right

now!" He poked the growing bulge between his brothers legs, making

Yukio blush furiously. He couldnt be getting hard for his brother! It

wasnt right! But it couldnt be denied. And before he knew it, his tail

was in the clawed hands of a sinful devil. It was being rubbed base to

end, driving Yukio crazy. His back arched and he kept gasping, never

seeming to be able to get an entire breath. "Thats it. Come on. For

me" Rin hissed lustfully. He forgot about his original plan simply to see if anthing was wrong with his brother. Now all he cared about was making Yukio into his toy. The mere thought gave his demon side a jolt of excitement.

More attention was given to the base and tip of the little black

tail. Whenever a finger would push in and rub, Yukio would jerk or

moan loudly. "Thats it. Feel good?" Rin asked softly as he put the tip

of the tail in his mouth. Yukio almost screamed, and whined louder.

"P-please...Rin! Stop! I beg you!" He started wiggling furiously until even Rin couldn't hold him down. Yukio broke free and ran out of the room in terror. He ran out to the

city. Being as far away from Rin as possible was his top priority at the moment.

It was very late at the True Cross park, but when everyone else left, Yukio managed to stay alone in the dark thanks to the cover of a tree. He sat under the low-branched fir tree to keep out of sight. As he sat there, tears began to fill his eyes. "How could Rin let his sinful nature take him over?" He asked aloud, "Now thanks to him, more demons have been invited into the city!" Yukio sniffled more. As strong as he was in battle, sex had always been his weak point. Even talking to girls in a cute lovey way made him nervous as ever. "Why cant I be stronger..." he whispered, "Why can't I help Rin with the simplest of things. Such as falling into his sinful nature...?" "Why do you want to?" answered an

unfamiliar voice. Yukio instinctively reached for his gun, only to

find, to his horror, that they werent there. For the first time since

becoming an exorcist he officially was out of moves.

He went on quoting scriptures desperately, only making the voice chuckle lowly.

"Whats the matter son? Are you scared of daddy?" It asked. Yukio's

eyes flung open and he nearly had a panic attack. Was it

really...satan? Of all demons Rin could have let in?! Yukio stood, only to find

himself face-to-face with his nightmares. A very beautiful man with

elegant horns and slender tail stood there. His eyes were bright red,

his claws long and black. His terrible smile held despicable pointed

teeth, his ears were long but somehow handsome. His revolting tongue

licked his lips as he gazed at his younger son. "Surprise, surprise!"

he laughed, "I managed to take on my demon form in this world. How can I possibly thank your brother enough for falling into his sinful nature?"

Yukio hissed and thrashed his tail. He was still very afraid, but angry at himself for ever letting Rin do such a thing. "Ah, so threatening" satan said as he played with a ball of flame between his clawed hands. "But I escaped Rin before he could do anything. I guess just the thoughts he had were enough to let a beast like you in" Yukio snapped. satan hissed with pleasure. "So it scared you enough to make you run like a sick coward? Oh, you know how much I love fear. Especially that of my little son" Said the demon lustily. "Dont talk to me like that! I dont care that you got here, but go back to hell!" Yukio growled. satan swung his head back in laughter then looked at his son with an evil smirk. He slowly paced toward Yukio, who in turn took steps back. "Dont be scared of daddy. He only wants you to be good" satan continued to say. He could feel fear eminating from Yukio, and it gave him strength.

In one swift, unexpected move, satan grabbed Yukio's throat and pushed

him against the tree. He didnt choke the little one, but his grip was

tight for sure. Yukio grabbed the strong arms and cried silently. He

was terrified. After locking his blood father out of Assiah for a

year, how would the terrifying demon dad pay him back now that Rin allowed him in again? With death? Or would he keep him alive? Yukio's mind swam as the grip got tighter. "P-Please daddy..." he whimpered. "Say it again!" demanded satan.

"Please daddy!" Yukio squeaked. satan laughed and let go of his neck, but pulled

Yukio's form next to his body.

"I never thought I'd hear you say that. Now how about giving you more a reason to beg?" He pushed Yukio's back against the tree and draped the fragile legs over his

broad shoulders. Yukio didnt like this position much, but if satan

pulled away, he would fall painfully to the ground. "Now I get to

satisfy my own sinful nature; oh how long ive waited!" satan said as

he extended his tongue. It licked Yukio's pants between his open legs, bringing an

erection against his will. "Dont do this to me! Stop!" "No, I want

to see you squirm and call me by the cute name you gave me" "I'm not

going to call you that..." The tongue lapped at the sensitive spot. "Oh, daddy! Please!" Yukio squealed with a jerk. The demon king chuckled and kept licking. One of his hands grabbed his son's furry tail. Yukio, having never really been exposed to this sort of attention, reacted the only way he knew how; like a child taking a punishment. He cried out and did his best to stop satans menacing actions. But no matter how hard he tried, satan kept touching his sensitive spots as if there were no resistance.

"Where do you think all the

incubus learned their craft? You should know that youre helpless

against the one who taught them" Yukio tried to make a serious face,

but there was no way in Assiah he could do it right now. Every time he

furrowed his brows sternly, they bended back into a worried-like

expression. Every time he tried to make a threatening growl, it turned

into a straining moan. At one point he even tried to say something

threatening, but his voice was weak and breathy. As the minutes passed

it only worsened.

"I'm surprised you can hold out this long! However,

even the strongest of wills can be broken, as youre about to see. Moan

my name; I dare you" satan hissed softly. By this time his tongue had

found its place on Yukio's shaft inside his pants, and his hands became more expert at

their dirty work on the outside. It was all the young one could do to not break into

a writhing fit. He was close; constantly getting closer. His reactions

worsened and his tolerance was failing. How long until satan got his

sinful desire answered? "I can feel you trembling. I can see your

sweat. Stop fighting it and let it take you!" satan growled

impatiently. Yukio was about to go over the edge when a 3rd person

entered the scene. It was Rin Okumura with his flaming sword drawn.

He swung the sword, causing minor damage and making satan back away.

Yukio fell to the ground in a shaking mess. Rin hurried next to him

and comforted his little brother. "Im so sorry for what I did Yuki. I

didnt have control over myself. And by letting my sinful nature take

me, I gave satan leeway to get to you. Im so sorry, you have no idea"

Yukio said nothing but the sweet childish hug he gave said all that

Rin needed. "Come now sons! Surely you cant forget your daddy's here,

now can you?" satan said nervously. He tried to lay a hand on Rin, but

pulled back in agony. His hand sizzled with an invisible burn. Rin stood holding his younger brother, who was not the least bit afraid now.

"I'm sorry 'daddy'" Rin responded, "But there are only two

powers at work here: Love and fear. Love casts out fear, and without

fear you are powerless! So run home back to hell, we dont want you

here" He helped Yukio back to the dorm with a smile. Now not having any strength and being subject to the person who let him in, satan had no choice but to go back to where he came from.

"Thank you for

being a great big brother" Yukio said as he gave a sweet grin, "Just

dont go crazy on me again" Rin nodded. "Of course. I promise not to

touch you again unless you want me to" Yukio looked off into space for

a moment in thought. "Actually," he said slowly, "As long as you aren't

rough or scare the jeezers out of me, you can touch me any time you

want. It did feel a bit nice when you tugged my tail" Now it was Rin's

turn to go off into thought. These words were only things he heard in

his big-brother fantasies when falling asleep in class. "I'll keep

that in mind. Be sure to watch your back. I'll come when you least

expect"


End file.
